The California serogroup viruses have become a serious cause of human disease, especially in the midwestern United States. Although the initial isolation of a member of this group, California Encephalitis, was from California, this group has been little studied in the western United States since that time other than in the central valley of California, and little information is available on the occurrence and public health importance of this important group of mosquito-borne pathogens. Preliminary studies indicate that in addition to California encephalitis, either Jamestown Canyon (JC), a newly emerging human pathogen, or Jerry Slough, a variety of JC originally isolated in California, also occur, especially in high mountain areas of the state. The relationship of JS to JC virus, antigenically and ecologically, is unknown. Northway, a bunyavirus belonging to the Bunyamwera serogroup, also may be a cause of human disease, and has been shown to occur in California as well. We feel that the varied climate and topography of California offers a unique opportunity to study vector relationships of these viruses. Specific objectives of this study are (1) to establish the prevalence of antibodies to California and Bunyamwera serogroup viruses in non-human vertebrates in mountainous areas of California, (2) to incriminate the principal vector species of California and Bunyamwera serogroup viruses in mountainous areas of California, (3) to determine the prevalence of antibodies to California and Bunyamwera serogroup viruses in humans in mountainous areas of California, (4) to document the presence, identity, and geographic and ecological distribution of California and Bunyamwera serogroup viruses in California.